Compact fluorescent lamps have been finding greater acceptance in both consumer and commercial lighting applications primarily because of their improved energy efficiency relative to conventional incandescent lamps and because of their longer life expectancy over the standard incandescent line of products. Though such products have been available in the marketplace for many years, early generation compact fluorescent lamps had suffered from certain deficiencies such as overall size and weight. These deficiencies have been eliminated recently by the introduction of shorter profile lamp envelopes that more readily fit within typical light fixtures and by the use of lighter, more compact electronic ballast circuits in place of conventional magnetic ballasts. One problem that remains to be solved is that of incorporating the increased life expectancy and energy efficiency of compact fluorescent lamps into a reflector type of lamp that is used extensively in recessed lighting and display lighting for instance. Presently, when a compact fluorescent lamp is combined with a reflector housing to achieve an efficient reflector lamp product, the overall size of such device is so large as to make this lamp impractical for most recessed lighting fixtures.
In addition to the need to improve the size and performance properties of a compact fluorescent version of a reflector lamp, to further improve the life expectancy of the compact fluorescent lamps in general, it has been proposed to provide an electrodeless version of a compact fluorescent lamp which could then apply to the reflection version thereof. By removing the electrodes from within the lamp envelope and exciting the discharge therein by means of an RF signal, the life expectancy can be increased significantly due to the elimination of a glass to metal seal around the electrodes and further due to the fact that ion emissions associated with the electrodes can be eliminated. An example of an electrodeless fluorescent lamp having an A-line configuration can be found in U.S. Pat. No. 4,010,400 in which it is disclosed that an ionizable medium can be disposed in a lamp envelope and excited to a discharge state by introduction of an RF signal in close proximity thereto such that by use of a proper phosphor, visible light can be produced by such discharge. In order to generate this RF signal, a ballast circuit arrangement can be disposed in the lamp base, such ballast circuit arrangement including a resonant tank circuit which utilizes a coil member extending into the lamp envelope to inductively couple the RF signal to the ionizable medium.
As with any conventional fluorescent lamp, an electrodeless discharge lamp will have a phosphor layer coated on the inner surface of the lamp envelope which is effective so as to enable conversion of the discharge from the ionizable medium into visible light. As to the phosphor material, it is the typical practice in fluorescent lamp manufacture to use halophosphates which are relatively inexpensive and are used extensively because of their good efficacy, low cost and wide range of acceptable colors. Although use of the halophosphate materials is appropriate for larger fluorescent lamps such as the conventional 2 and 4 foot versions, in a compact fluorescent lamp application it is necessary to utilize comparatively more expensive rare earth phosphors. Given this fact, in order to achieve a cost effective replacement for a conventional incandescent type reflector lamp that utilizes electrodeless fluorescent technology, it would be advantageous if a coating arrangement could be developed that minimized the usage of the expensive rare earth phosphors in terms of the applied thickness of such materials.
In addition to the requirement of developing a phosphor coating arrangement that utilizes the rare earth phosphors in a cost effective manner, there is the requirement that for a reflector version of an electrodeless compact fluorescent lamp, a deposition of a reflector coating be applied in a manner that results in a maximum light output through the face region of the lamp envelope. Such an electrodeless fluorescent reflector lamp presents a special difficulty; that is, how to deposit the reflector coating in cooperation with the phosphor coating. It is known that finely divided titania can be used as the reflective material and can be applied to the lower portion of a lamp envelope which is shaped substantially like a conventional reflector lamp. The visible reflectivity of such coating should be as close to 1 as possible which would require a fairly thick coating of between 50-500 particle layers of the reflecting material.
It is not as straightforward to determine the coating thickness distribution of the phosphor material. For example, most aperture fluorescent lamps such as are used in reprographic equipment, have no phosphor coating on the window; such window as would correspond to the face region of a reflector lamp. This has the disadvantage that UV radiation emitted by the discharge is absorbed by the glass without being converted to visible light.
Alternatively, the phosphor coating can be applied to the entire interior surface of the lamp envelope to ensure maximum conversion to visible light. Using conventional techniques, this could be accomplished by filling the lamp envelope with a suspension containing the phosphor powder and then draining or alternatively, flushing a suspension into the lamp envelope. Either method will give a phosphor coating weight distribution which is thicker on the face and thinner on the lower region of the envelope due to the characteristics of gravity induced draining. Typically, when the suspension used is thick enough to produce a good phosphor coating for absorbing UV, the coating on the face is so thick that it actually reflects visible light. By reflecting visible light from the face region of a reflector lamp, a significant amount of light is trapped within the lamp and will undergo multiple reflections causing light loss. Furthermore, a significant amount of trapped light is lost by absorption by mercury deposits, impurities, and transmission through the reflecting portions of the lamp. Accordingly, it would be advantageous if a phosphor coating weight distribution could be developed which would allow for efficient conversion of UV into light output yet would not be so thick as to reflect a significant amount of light back away from the face region of the envelope.
For a conventional electroded compact fluorescent application the development of a coating arrangement that varied the thickness would not be practical given the typical geometric configuration of the lamp envelope. Such limitation is not a factor in an electrodeless fluorescent lamp in general and a reflector version in particular however given that there is a variation in the diametric dimension of the lamp envelope in order to accommodate the re-entrant cavity. Accordingly, it would be possible to utilize a combination of varying thicknesses of the rare earth phosphors in order to achieve a reflector lamp that would be of a minimum size and would provide a maximum amount of light output.
One problem with providing a phosphor coating arrangement having varying thicknesses at different areas of the lamp envelope is in the implementation of a coating method which would be applicable to high speed automated manufacturing systems where it is necessary to provide for a high quality product having uniform physical characteristics for sales quantities projected to be in the millions of units. Moreover, it is also necessary that such manufacturing method achieve the end product in as simple and cost effective manner as possible without requiring the addition of costly equipment modifications to existing equipment presently used in the manufacture of fluorescent lamps. Accordingly, it would be advantageous if a manufacturing method could be developed that allowed for the implementation of the varying thickness phosphor coating of a reflector type lamp which utilized an electrodeless fluorescent lamp as the light source.